This invention relates to a fuel injector for use in supplying fuel, under pressure, to a combustion space of a compression ignition internal combustion engine.
In order to reduce emissions levels, it is known to provide fuel injectors in which the total area of the openings through which fuel is delivered can be varied, in use. One technique for achieving this is to use two valve needles, an outer valve needle which is slidable within a bore formed in a nozzle body and an inner valve needle which is slidable within a bore provided in the outer valve needle. Movement of the outer valve needle controls the supply of fuel to some of the outlet openings formed in the nozzle body, whereas movement of the inner valve needle controls the supply of fuel to other outlet openings formed in the nozzle body.
European patent application EP 99304430.4 describes a fuel injector of the aforementioned type in which movement of both the inner and outer valve needles is controlled by a single actuator. The bore in the outer valve needle is provided with a step which is engageable with an enlarged upper end region of the inner valve needle. In use, when the outer valve needle is lifted in an upwards direction away from a first valve seating by a relatively small amount, a first set of outlet openings are exposed, causing fuel to be ejected therefrom. Upward movement of the outer valve needle by a further amount causes the inner valve needle to engage the step such that movement of the outer valve needle is transmitted to the inner valve needle. This causes the inner valve needle to lift away from a second seating, exposing a second set of outlet openings from which fuel is then ejected. This provides a higher fuel delivery rate.
It is an object of the invention to provide a fuel injector in which movement of the outer valve needle is transmitted to the inner valve needle by alternative means.
According to the present invention there is provided a fuel injector comprising an outer valve needle provided with a bore, an inner valve needle slidable within the bore, the outer and inner valve needles being engageable with first and second seatings respectively to control the supply of fuel from the fuel injector, the inner valve needle having a passage formed therein communicating with a sac chamber located downstream of the second seating, whereby movement of the outer valve needle away from the first seating causes fuel to flow through the passage into the sac chamber such that fuel pressure within the chamber causes the inner valve needle to lift away from the second seating.
Conveniently, initial movement of the outer valve needle away from the first seating by a relatively small amount to a first fuel injecting position causes a first set of outlet openings to be exposed from which fuel is then ejected, and further movement of the outer valve needle to a second fuel injecting position causes inner valve needle movement away from the second seating, thereby causing a second set of outlet openings to be exposed from which fuel is then ejected. By transmitting movement of the outer valve needle to the inner valve needle solely by increasing the fuel pressure within the sac chamber, the fuel injector is caused to move into a relatively stable second fuel injecting position. This provides an advantage over known fuel injectors in which the second fuel injecting position can be relatively unstable.
Conveniently, the outer valve needle is slidable within a further bore provided in a nozzle body and the inner valve needle may be provided with an axially extending passage communicating with one or more radially extending passages provided in the inner valve needle, movement of the outer valve needle within the further bore away from the first seating causing the end of the or each radially extending passage to be exposed to permit fuel to flow into the sac chamber.